The present invention relates to a hydraulic impact tool adapted to be mounted on the head of a hydraulic power shovel or the like and used to demolish a concrete structure, to crush rocks, to excavate a rock base, or the like.
Hydraulic impact tools can be classified roughly into an accumulator type and a gas pressure type.
With an accumulator type tool, pressurized oil is accumulated in an accumulator while a piston is rising and is released during its downward stroke to accelerate the piston.
With a gas pressure type tool, one example of which is disclosed in the Japanese Patent Publication No. 54-32192, a piston compresses a gas filled in the space above the piston to store energy when it rises under oil pressure. During its downward stroke, the compressed gas expands to accelerate the piston. The impact tool disclosed in the abovesaid Publication is shown in FIG. 6 in which numeral 1 designates a cylinder having a tool 2 such as a chisel slidably mounted in the lower end thereof.
A piston 4 formed with a large-diameter portion 3 is mounted in the cylinder 1 to strike the tool 2. The cylinder 1 has an upper chamber 5 charged with gas over the piston 4 to exert the gas pressure to the piston 4 as it reaches its upper limit.
The piston 4 has small-diameter portions over and under the large-diameter portion 3. A middle chamber 6 and a lower chamber 7 are formed between the small-diameter portions and the inner periphery of the cylinder 1.
A valve chest 8 is formed at one side of the cylinder 1. A valve body 10 formed with a center bore is mounted in the valve chest 8. The valve chest communicates with the cylinder 1 through oil channels extending from the upper and lower parts of the former to the upper part of the middle chamber 6 and to the lower part of the lower chamber 7, respectively. Further, the cylinder 1 and the valve chest 8 have their respective mid-portions communicating with each other by means of one main oil channel and a branch channel.
The valve chest 8 has its upper and lower parts connected to a discharge port 11 and an oil feed port 12, respectively. From the oil feed port 12, another oil channel branches and leads to the top end of a plunger 13 for pressing down the valve body 10.
In operation, when the valve body 10 is at its lower limit, pressure oil is supplied through the oil feed port 12 to pressurize the lower chamber 7. Since the middle chamber 6 is open to the discharge port 11, the piston 4 rises up the cylinder to compress the gas in the upper chamber 5.
When the piston 4 approaches the uppermost position, the oil feed port 12 gets into communication with the middle oil channels through which pressure oil flows into the valve chest 8 to push up the valve body 10. As soon as the valve body 10 clears the bottom of the valve chest 8, the lower chamber 7 communicates with the discharge port 11 through the bore in the valve body 10. Thus, the piston 4 is pushed down by the pressure of gas in the upper chamber 5 to strike the tool 2.
With this prior art impact tool, when the piston 4 rebounds violently immediately after striking the tool 2, the pressure in the lower chamber 7 drops sharply because the chamber 7 is open to the discharge port 11, thus allowing air bubbles in the hydraulic oil to grow rapidly. This phenomenon is called cavitation. When the valve body 10 descends thereafter and pressure oil flows back into the lower chamber 7, the air bubbles which have grown large collapse in an instant, producing a very high pressure and a shock wave. This happens repeatedly several hundred times a minute. Thus, the piston 4 and the cylinder 1 tend to develop erosion on their surface after long use.